Metal halide lamps have been used as a light source of a lighting system in indoor and outdoor facilities. Recent years have seen the introduction of so-called ceramic metal halide lamps with an arc tube envelope made of a ceramic material.
With an envelope made of a ceramic material, the heat-resistance improves as compared with a conventional metal halide lamp having an envelop made of quartz glass. In addition, the ceramic material undergoes less reaction with a metal halide filled within the arc tube, so that the envelop can withstand a heavier wall load. As a result, ceramic metal halide lamps achieve higher lamp efficiency as compared with metal halide lamps having a quartz glass envelope. Hereinafter, a ceramic metal halide lamp is simply referred to as a “lamp”, unless in the case where the lamp needs to be distinguished from a metal halide lamp with a quartz glass envelope.
In order to further improve the lamp efficiency, it is suggested to fill the envelope with metal halides including lanthanum series halides, such as cerium (Ce) and praseodymium (Pr), along with sodium halides (Na). It is also suggested to use a relatively narrow arc tube (satisfying L/D>4, where L denotes the inter-electrode distance, and D denotes the inner diameter of the arc tube)(See, for example, patent literature 1). The lamp described above is said to achieve high efficiency of 111-177 (lm/W).
Here, a description of the basic lamp structure is given.
As illustrated in FIG. 1, the lamp includes: an outer tube 3 that is closed at a first end and sealingly attached to a flare 2 at a second end; two power supply lines 4 and 5 that are partly buried within the flare 2 so as to place one end of each power supply line within the outer tube 3; an arc tube 6 supported within the outer tube 3 by the power supply lines 4 and 5; and a base 7 fixed to the second end of the outer tube 3.
The power supply lines 4 and 5 are connected to the base 7 and feed power supply received via the base 7 from an external source to a pair of electrodes disposed within the arc tube 6.
Note that the outer tube 3 is maintained under vacuum (reduced pressure) and that the arc tube is filled with metal halides and starting buffer gas.
Now, a discussion is given to lamp lighting devices for operating a lamp. In recent years, there is a greater demand for a lamp lighting device to be smaller and lighter, and have more sophisticated functions. In response to this demand, electronic-type lamp lighting devices are replacing magnetic-type lamp lighting devices. Generally, electronic-type lamp lighting devices for lamps employ the square-wave lighting method with the aim to avoid a phenomenon called “acoustic resonance”, in which the lamp flickers when the frequency approaches a specific value.
According to the square-wave lighting method, the lamp current is limited within a high-frequency range to reduce the current components in size. In addition, the high frequency current is reversed in polarity within a low-frequency range in which no acoustic resonance is caused. Then, the high-frequency components are removed by a filter circuit. Thus, the square-wave current composed exclusively of low-frequency components is supplied to the lamp. In this way, the lamp is stably operated, while avoiding acoustic resonance.
Generally, the lamp voltage tends to increase with the passage of time after the initial stage of lamp operation. With the magnetic-type lamp lighting device, it is normally true that the lamp voltage required for re-starting the lamp operation increases with the increase in lamp voltage. Eventually, the lamp operation can no longer be maintained and the discharge fades out. On the other hand, an electronic-type lamp lighting device is capable of-lamp power control, so that the risk of discharge fading-out is smaller even after some duration of lamp operation.
Patent Literature 1: JP Patent Application Publication No. 2000-501563